This invention generally relates to variable capacity vapor compression refrigeration systems, and more specifically to a fluid flow control device particularly well suited for use with such refrigeration systems.
Vapor compression refrigeration systems generally comprise a compressor, a condenser, expansion means, and an evaporator connected together by appropriate refrigerant lines to form a closed loop refrigeration circuit. Refrigerant vapor is compressed by the compressor and fed to the condenser where the refrigerant releases heat to a cooling medium and condenses. The condensed refrigerant then flows through the expansion means, reducing the pressure of the refrigerant. From the expansion means, the refrigerant passes into the evaporator, absorbs ambient heat, and vaporizes. Vaporous refrigerant is then drawn back into the compressor, completing the circuit.
Refrigeration systems of the foregoing type are often used in situations such as commercial and industrial buildings where the load upon the system may vary over a wide range. Commonly, the refrigeration system is designed to meet the maximum load to which it may be subjected, and when the system operates under lower load conditions, the capacity of the refrigeration system is reduced. Frequently, such a refrigeration system has a variable capacity compressor, and system capacity is reduced by reducing the compressor capacity. For example, if the refrigeration system has a reciprocating piston type compressor, system capacity may be reduced by partially or completely unloading one or more of the piston cylinders of the compressor.
Variable capacity refrigeration systems generally include an evaporator having a plurality of groups of refrigerant circuits and a plurality of evaporator inlet lines for conducting refrigerant to the evaporator circuit groups. Each group of circuits is supplied with refrigerant from a different one of the evaporator inlet lines and; under low load operating conditions, when the capacity of the refrigeration system is reduced, one or more of these evaporator inlet lines is closed, removing from operation the evaporator circuit group or groups fed by the closed inlet line or lines. As is understood in the art, removing selected evaporator circuits from operation in response to reduced system capacity improves system performance by, inter alia, maintaining high fluid velocities through the evaporator and thus preventing an excessive accumulation of lubricant therein.
Heretofore, solenoid operated valves have usually been used to close the selected evaporator inlet lines. To elaborate, one or more of the evaporator inlet lines of a refrigeration system are each provided with a valve, with each of these valves connected to a different electrically operated solenoid. Normally, the valves are all in open positions, allowing fluid flow through the selected evaporator inlet lines. By activating the solenoids, the valves are moved to closed positions, preventing fluid flow through the selected evaporator inlet lines. The valves are returned to their open positions by deactivating the solenoids. These solenoids, in turn, are activated by a thermal or pressure sensitive switch that senses a parameter indicative of the operating condition of the refrigeration system and that closes, and thus activates the solenoids, when the capacity of that refrigeration system is reduced below a predetermined level.
While these prior art solenoid arrangements are very effective, they are also relatively expensive. In particular, the solenoid valves themselves are comparatively expensive items. In addition, the control for the solenoids, usually including at least one electrically conductive thermal or pressure sensitive switch, also represents a significant cost.